Abstract-Previous studies have shown that mitofusin 2 (Mfn-2) (or hyperplasia suppressor gene [HSG]) inhibits vascular smooth muscle cell (VSMC) proliferation. Here, we demonstrate that Mfn-2 is a primary determinant of VSMC apoptosis. First, oxidative stress with H 2 O 2 , inhibition of protein kinase C with staurosporine, activation of protein kinase A with forskolin, and serum deprivation concurrently elevate Mfn-2 expression and induce VSMC apoptosis. Second, overexpression of Mfn-2 also triggers apoptosis of VSMCs in culture and in balloon-injured rat carotid arteries, thus contributing to Mfn-2-mediated prevention of neointima formation after angioplasty. Third, Mfn-2 silencing protects VSMCs against H 2 O 2 or Mfn-2 overexpression-induced apoptosis, indicating that upregulation of Mfn-2 is necessary and sufficient for oxidative stress-mediated VSMC apoptosis. The Mfn-2 proapoptotic effect is independent of its role in mitochondrial fusion but mainly mediated by inhibition of Akt signaling and the resultant activation of the mitochondrial apoptotic pathway, as manifested by decreased Akt phosphorylation, increased mitochondrial Bax/Bcl-2 ratio, cytochrome c release, and activation of caspases-9 and caspase-3. Furthermore, Mfn-2-induced apoptosis was blocked by overexpression of an active phosphoinositide 3-kinase mutant or Bcl-xL or inhibition of caspase-9 but not caspases-8. Key Words: PI3K-Akt Ⅲ apoptosis Ⅲ HSG Ⅲ Mfn-2 Ⅲ vascular smooth muscle cells A poptosis is a programmed cell death that is essential for embryonic development and for tissue homeostasis, remodeling, and immune responses. There are 2 major apoptotic signaling cascades: the first is the death receptor (Fas or tumor necrosis factor receptor)-mediated pathway involving activation of caspase-8 and its downstream executioner caspases; the other is the mitochondrial pathway activated by cellular deprivation or stress, and it involves sequentially the release of cytochrome c, the recruitment of apoptotic protease activating factor-1 (Apaf-1), and the activation of caspase-9 and downstream executioner caspases. [1][2][3] Defects (inhibition or exacerbation) of either pathway trigger proliferative or degenerative disorders, including atherosclerosis, restenosis, myocardial infarction, cancers, neurodegenerative diseases, and AIDS. [3][4][5][6][7] Ras, a small GTPase, plays a central role in the regulation of many fundamental biological processes, such as cell proliferation, differentiation, senescence, survival, and growth via activation of a wide array of downstream signaling pathways. Among them, the Ras-Raf-MEK-ERK/ mitogen-activated protein kinase (MAPK) pathway and the Ras-PI3K-Akt (also known as protein kinase [PK]B) pathway are vital for cell proliferation and cell survival. 8 -11 Whereas the Ras-MAPK pathway drives cell cycle progression, 9 the activation of the Ras-PI3K-Akt signaling blocks apoptotic cell death. 11 In particular, Akt-mediated phosphorylation of proapoptotic members of the Bcl-2 family, including Bad and Bax, prevents ...
